1. Field of the Invention
The present invention relates to a semiconductor memory device and a method for producing the same, more particularly relates to a dynamic random access memory (DRAM) or other semiconductor memory device having a storage node electrode and a method for producing the same.
2. Description of the Related Art
In recent very large scale integrated circuits (VLSI) and other semiconductor devices, reductions of scale of 70% have been realized in three years. Higher degrees of integration and higher performance have been achieved. For example, DRAMs have a memory cell structure having a switching use metal-oxide-semiconductor field effect transistor (MOSFET) and a memory capacitor. They have become increasingly miniaturized and reduced in size in recent years and have become increasingly large in capacity and higher in degree of integration as process drivers in semiconductor devices, for example, with DRAMs having a storage capacity of 1 Gb being announced at the academic level. Along with the miniaturization, the area of the memory cells has been reduced and the area occupied by the memory capacitors has been reduced.
However, in order to secure a sufficient operating margin and secure a tolerance to soft error due to alpha rays so as to raise the reliability of the stored data, the storage capacity Cs of the memory capacitor is held at a constant value of 20 to 30 fF per bit regardless of the generation of the DRAM.
Accordingly, irrespective of the fact that the area occupied by the memory capacitor has been reduced along with the miniaturization, the required amount of storage capacity Cs must be secured. Various attempts have been made to deal with this.
For example, other than the method of reducing the thickness of a capacitor insulating film so as to increase the storage capacity, a method has been developed of using as the capacitor insulating film tantalum oxide (Ta2O5), BST, STO, or the like having a high dielectric ratio in place of an ON film (or ONO film) comprising a composite film of a silicon nitride film and a silicon oxide film so as to improve the materials comprising the capacitor insulating film and increase the storage capacity of the capacitor.
On the other hand, improvements have been made to the electrode structure of the capacitor as well. Capacitors having various structures have been developed. A memory capacitor comprises a storage node electrode (electrode connected to the transistor of the capacitor), a plate electrode (grounded electrode of the capacitor), and a capacitor insulating film between them. By increasing the surface area of the storage node electrode and the plate electrode, it is possible to increase the storage capacity of the capacitor.
A planer type having a planar structure has been used in the related art, but at present, generally the storage node electrode is made three-dimensional to obtain a more complex shape, the side wall surfaces of the storage node electrode etc. are utilized, and therefore the surface area of the storage node electrode is increased to increase the storage capacity without increasing the occupied area of the capacitor. As a three-dimensional storage node electrode, there are for example a stack type, a trench type, etc.
In the trench type, the storage node electrode is formed in the depth direction with respect to the substrate, so it is necessary to investigate the adverse influence due to digging into the substrate. On the other hand, the stack type can be classified into two types, that is, the capacitor-over-bit line (COB) and the capacitor-under-bit line (CUB) type. Among them, in the case of the COB stack type, since the capacitor (storage node electrode) is formed after the bit line, there is the advantage that the largest capacitor (storage node electrode) determined by the miniaturization can be formed in the cell region.
As the COB stack type described above, a variety of types have been developed such as the pedestal stack type, fin type, and cylinder type (crown type). As the cylinder type, other than a type having a single cylindrical part, a type having a double cylindrical part has been developed. Further, similarly, a method of coarsening the storage node electrode surface in order to increase the surface area and a method of controlling the polycrystalline silicon electrode-forming temperature to providing semispherical roughness on the surface have been developed.
Among them, the cylinder type storage node electrode can make effective use of the peripheral length of the electrode as the surface area, therefore, despite the reduction of the occupied area, the storage capacity is easily secured. This makes it one of the electrode structures best suited to the miniaturization, increase of degree of integration, and reduction of size of semiconductor memory devices. The methods for forming cylinder type storage node electrodes may be roughly classified into methods of forming a side wall-shaped electrode in the side wall portion of for example a recessed type oxide film and methods of forming the electrode material at the inner wall of a recessed type oxide film. In general, the latter enables a larger exposure margin in lithography and a larger margin with respect to the focus depth, so is advantageous with respect to further miniaturization.
The methods of forming cylinder type storage node electrodes of the related art, however, suffer from several problems such as a difficulty in exposing the top face of the storage node contact plug and connection failure between the storage node contact plug and the storage node electrode and, even when there is no connection failure, inadvertent etching of the inter-layer insulating film and, in the worst case, a short circuiting of the bit line and the storage node. These will be explained in further detail later, along with the method of production of the related art, with reference to the attached drawings.
An object of the present invention is to provide a DRAM or other semiconductor memory device having a memory capacitor wherein provision is made of a high quality storage node electrode and storage node contact plug enabling prevention of the connection failure between the storage node contact plug and the storage node electrode and prevention of the removal of the insulating film below the etching stopper which is a cause of short-circuiting and a method for producing the same.
To attain the above object, according to a first aspect of the present invention, there is provided a semiconductor memory device comprising a plurality of memory cells each having a memory capacitor having a storage node electrode and a transistor, comprising a substrate, a transistor formed on said substrate, a first insulating film formed on said substrate covering said transistor, a storage node contact hole formed in said first insulating film and reaching a source and drain region of said transistor, a storage node contact plug buried in said storage node contact hole, a storage node electrode formed connected to said storage node contact plug, a second insulating film formed above said first insulating film in the gap portion of said storage node electrodes, a capacitor insulating film formed above said storage node electrode, and a plate electrode formed above said capacitor insulating film, said storage node electrode and said storage node contact plug being formed connected at least at part of the top surface and side surface of said storage node contact plug.
That is, according to the semiconductor memory device of this aspect of the present invention, the storage node electrode and the storage node contact plug are formed connected at least at part of the top surface and the side surface of the storage node contact plug. In the related art, they were connected at only the top surface of the storage node contact plug, therefore when the plug loss was large, it was sometimes difficult to sufficiently expose the top surface of the storage node contact plug. By connecting them at part of the top surface and the side surface of the storage node contact plug, it becomes possible to reliably connect the storage node electrode and the storage node contact plug.
In the semiconductor memory device of this aspect of the present invention, preferably said storage node electrode and said second insulating film are formed in contact at least at part of the top surface and the side surface of said second insulating film. Due to this, it becomes possible to bring the storage node electrode and the second insulating film in contact without generating a gap. When the insulating film used as the mold for forming the storage node electrode is removed with a selectivity with respect to the second insulating film, the removal of the first insulating film below the second insulating film can be prevented.
Further, to achieve the object, according to a second aspect of the present invention, there is provided a semiconductor memory device comprising a plurality of memory cells each having a memory capacitor having a storage node electrode and a transistor, comprising a substrate, a transistor formed on said substrate, a first insulating film formed on said substrate covering said transistor, a storage node contact hole formed on said insulating film and reaching the source and drain region of said transistor, a storage node contact plug buried in said storage node contact hole, a storage node electrode formed connected to said storage node contact plug, a second insulating film formed above said first insulating film in the gap of said storage node electrode, a capacitor insulating film formed above said storage node electrode, and a plate electrode formed above said capacitor insulating film, said storage node electrode and said second insulating film being formed in contact at least at part of the top surface and the side surface of said second insulating film.
According to the semiconductor memory device of this aspect of the present invention, since the storage node electrode and the second insulating film are formed in contact at least at part of the top surface and the side surface of the second insulating film, it becomes possible to bring the storage node electrode and the second insulating film into contact without generating a gap. When the insulating film used as a mold for forming the storage node electrode is removed with a selectivity with respect to the second insulating film, the removal of the first insulating film below the second insulating film can be prevented.
In the semiconductor memory device of this aspect of the present invention, preferably the bit line is formed above said transistor via the insulating film and said transistor and said bit line are covered by said first insulating film. As the COB type capacitor (storage node electrode), the largest capacitor (storage node electrode) determined by the miniaturization can be formed on the cell region.
In the semiconductor memory device of this aspect of the present invention, preferably said storage node electrode is a cylinder type. In the cylinder type, the peripheral length of the electrode can be effectively used as the surface area, therefore the storage capacity can be easily secured even while reducing the occupied area thereof.
Further, to attain the above object, according to a third aspect of the present invention, there is provided a method for producing a semiconductor memory device comprising a plurality of memory cells each having a memory capacitor having a storage node electrode and a transistor, including a step of forming a transistor on a substrate, a step of forming a first insulating film covering said transistor, a step of opening a storage node contact hole reaching a source and drain region of said transistor in said first insulating film, a step of forming a storage node contact plug by filling said storage node contact hole by a conductor, a step of forming a second insulating film having a different etching selectivity from that of said first insulating film above said first insulating film, a step of forming a third insulating film having a different etching selectivity from that of said second insulating film above said second insulating film, a step of opening an opening portion acting as the mold for forming the storage node electrode and exposing at least the top surface of said storage node contact plug in said second insulating film and said third insulating film, a step of etching to selectively remove said first insulating film and said third insulating film with respect to said second insulating film to cause the bottom surface and the side wall surface of said opening portion to retract, a step of forming the storage node electrode connected to said storage node contact plug while bringing it in contact with said second insulating film by using said opening portion as a mold, a step of removing said third insulating film by using said second insulating film as an etching stopper, a step of forming a capacitor insulating film above said storage node electrode, and a step of forming a plate electrode above said capacitor insulating film.
That is, in the method for producing a semiconductor memory device of this aspect of the present invention, a transistor is formed on a substrate, the first insulating film is formed covering the transistor, a storage node contact hole reaching the source and drain region of the transistor is opened in the first insulating film, and the storage node contact hole is filled by the conductor to form a storage node contact plug.
Next, a second insulating film having a different etching selectivity from that of the first insulating film is formed above the first insulating film, and a third insulating film having a different etching selectivity from that of the second insulating film is formed above the second insulating film.
Next, in the second insulating film and the third insulating film, an opening portion acting as the mold for forming the storage node electrode and for exposing at least the top surface of the storage node contact plug is opened and the bottom surface and the side wall surface of the opening portion are caused to retract by etching for selectively removing the first insulating film and the third insulating film with respect to the second insulating film.
Next, a storage node electrode is formed connected to the storage node contact plug while bringing it in contact with the second insulating film by using the opening portion as a mold, a third insulating film is removed by using the second insulating film as an etching stopper, a capacitor insulating film is formed above the storage node electrode, and a plate electrode is formed above the capacitor insulating film.
According to the method for producing a semiconductor memory device of this aspect of the present invention, an opening portion acting as the mold for forming the storage node electrode is opened in the second insulating film and the third insulating film, then the bottom surface and the side wall surface of the opening portion are made to retract, whereby it becomes possible to cause said bottom surface to retract so that the storage node contact plug projects out inside the opening portion and the storage node electrode and the storage node contact plug can be connected at least at part of the top surface and the side surface of the storage node contact plug. By this, it becomes possible to reliably connect the storage node electrode and the storage node contact plug.
Further, it becomes possible to cause the side wall surface to retract so that said second insulating film projects out into the opening portion at the side wall surface and possible to bring the storage node electrode and the second insulating film into contact without a gap and, when removing the third insulating film using the second insulating film as an etching stopper, to prevent the removal of the first insulating film below the second insulating film.
In the method for producing a semiconductor memory device of this aspect of the present invention, preferably, in the step of causing the bottom surface and the side wall surface of said opening portion to retract, said first insulating film and said third insulating film are selectively removed with respect to the second insulating film by isotropic etching. According to the isotropic etching, the bottom surface and the side wall surface of the opening portion can be made to retract.
In the method for producing a semiconductor memory device of this aspect of the present invention, preferably, in the step of forming said storage node contact plug, said storage node contact plug is formed so that the height of the top surface of said storage node contact plug matches the height of the surface of said first insulating film. Alternatively, preferably, the step of forming said storage node contact plug includes a step of forming a conductor on the entire surface while filling said storage node contact hole and a step of removing said conductor formed on the outside of said storage node contact hole by polishing. By this, a storage node contact plug can be formed while suppressing the plug loss.
In the method for producing a semiconductor memory device of this aspect of the present invention, preferably the step of forming said storage node electrode includes a step of forming a storage node electrode use layer connected to said storage node contact plug while bringing it in contact with said second insulating film by using said opening portion as a mold, a step of forming a fourth insulating film above said storage node electrode use layer, and a step of dividing said storage node electrode use layer into individual storage node electrodes by polishing from the top surface of said fourth insulating film, and in the step of removing said third insulating film by using said second insulating film as an etching stopper, said fourth insulating film is simultaneously removed. It is possible to use the third insulating film and the fourth insulating film as layers for forming the storage node electrode and processing the same.
In the method for producing a semiconductor memory device of this aspect of the present invention, preferably said first insulating film and said third insulating film are formed by silicon oxide and said second insulating film is formed by silicon nitride. Due to this, the bottom surface and the side wall surface of the opening portion acting as the mold of the storage node electrode can be caused to retract by removing the first insulating film and the third insulating film by etching with selectivity with respect to the second insulating film. Further, the third insulating film can be removed by using the second insulating film as an etching stopper.